This invention relates to a thin-film forming apparatus for use in the fabrication of electronic devices, semiconductors, etc. More particularly, this invention relates to a thin-film forming apparatus that is capable of consecutively measuring the composition of a sample in the process of forming the desired thin film on the sample. This invention also relates to a method and apparatus for analyzing a plurality of elements that are present on the surface of a material of interest or in its neighborhood, as well as a thin-film forming apparatus that is capable of measuring the composition of a sample during thin film formation in the process of semi-conductor fabrication.
As the scale of integration of electronic devices, semiconductor devices, etc. has increased while their feature size has decreased today, it has become apparent that the performance of those devices tends to depend on the quality of the thin films they use. For example, if the silicon (Si) to oxygen (O) ratio in a silicon dioxide (SiO.sub.2) deviates from the stoichiometric value of 1:2, one may conclude that an unwanted substance other than SiO.sub.2 has entered the film to change its dielectric constant, whereby desired high-frequency characteristics cannot be attained. Further, when forming a thin compound film, it is often difficult to attain the desired composition primarily due to the differences in sputtering yield and sticking coefficient. Conventionally, a composition analyzer is used to evaluate the composition of a thin film after making it with a thin-film forming apparatus. But, in this case, it has been difficult to form the thin film without any change in its composition due to variations in the process conditions being properly corrected during thin film formation. Under these circumstances, attempts have been made to consecutively measure the change that will occur in the composition of the thin film being formed.
FIGS. 17 and 18 are diagrammatic views of the prior art thin-film forming apparatus that is described in Ino et al., "Surface Analysis by RHEED Excited Total Reflection Angular X-Ray Spectroscopy (TRAXS)" in Oyo Butsuri (Applied Physics), 56, 7, 1987, pp. 843-850. FIG. 17 shows the apparatus as it is equipped with an elemental analyzer for measuring the compositional changes of a sample. Shown by 1 is a vessel for keeping an X-ray detector in vacuum; 3 is a window that maintains the vacuum enclosure of the X-ray detector but which admits the passage of X-rays; 6 is a liquid nitrogen dewar vessel for cooling the X-ray detector; 7 is an X-ray transmitting window; 10 is a thin-film forming vacuum chamber; 12 is a sample making mechanism; 13 is an exhaust unit for degasifying the vacuum chamber 10; 21 is a sample; and 31 is an excitation source that excites the elements in the sample 21 to emit their characteristic X-rays (fluorescent X-rays). Shown by 51 is a sample holder with a stage 52 for mounting the sample 21 in position.
FIG. 18 shows the interior of the vessel 1 in detail. Shown by 2 is the X-ray detector (hereunder sometimes referred to simply as the "detector"); 4 is an amplifier for amplifying the output signal from the detector 2; and 5 is a rod that guides the signal from the amplifier 5 to the outside of the vacuum enclosure and which lets in a cooling medium for cooling the detector.
The operation of the apparatus shown in FIGS. 17 and 18 is described below. To form a thin film of gallium arsenic (GaAs), he activates the mechanism 12 is activated to have gallium (Ga) and arsenic (As) evaporated onto the sample 21. During the thin-film formation, an excitation beam such as an electron beam generated from the excitation source 31 such as an electron gun is incident upon the sample 21, whereby Ga and As are excited to emit their characteristic X-rays. The emitted X-rays exit from the chamber 10 through the window 7 and travel in air atmosphere to be admitted into the detector 2 through the window 3 at the front of the vessel 1. Signals associated with the detected characteristic X-rays are amplified by the amplifier 4 and the amplified signals pass through the rod 5 to be fed into a spectrum analyzer (not shown) or some other suitable external circuit. With this arrangement, the composition of the surface of sample 21 can be analyzed while a thin film is formed on the sample.
The prior art elemental analyzer is described below in detail. FIG. 19 is a diagrammatic view of the elemental analyzer described in Japanese Patent Public Disclosure No. 82840/1985. As shown, the analyzer comprises the detector 2, a vacuum chamber 11, an exhaust port 14 which is connected to a vacuum pump (not shown) that is operated to keep the inside of the chamber 11 in vacuum, an electron gun 32 for emitting an electron beam onto the sample 21, the sample holder 51, the sample mounting stage 52, a spectrum analyzer 71 for spectrum separating the X-rays detected with the detector 2, and a memory unit 72 for storing the output of the spectrum analyzer. The memory unit 72 may be replaced by a display unit.
The operation of the elemental analyzer shown in FIG. 19 is described below. The sample 21 is to be mounted on the stage 52 at the distal end of the sample holder 51. In order to facilitate its mounting, an opening/closing portion is provided in a suitable area of the vacuum chamber 11 so that the sample 21 can be pushed into the chamber 11 via said portion. Alternatively, the vessel 11 may be so constructed that the sample holder 51 can be pulled out for mounting the sample 21 outside the vessel. The sample holder 51 is equipped with bellows or some other suitable means for permitting it to rotate or move back and forth so that the position and inclination of the sample 21 can be freely adjusted.
In response to a current from a power source, the electron gun 32 generates and emits an electron beam. The emitted electron beam is focused by an optional electronic lens and a collimator (not shown) to be incident on the surface of the sample 21. This electron beam excites the elements that are present on the surface of the sample 21 and in its neighborhood, whereby X-rays characteristic of the excited elements are emitted for detection with the detector 2. The detected X-rays are spectrum separated with the spectrum analyzer 71 and the X-ray spectra supplied from the spectrum analyzer 71 are stored in the memory unit 72.
The angle at which emitted X-rays are picked up is selected at the angle of total reflection of electromagnetic waves (generally characteristic X-rays) emitted from the sample 21, namely the critical angle, or at nearby angles. The reason for selecting the critical angle as the angle at which emitted X-rays are to be picked up is described below on the basis of the disclosures in "Oyo Butsuri (Applied Physics)", 56, 7, 1987, pp. 842-850 and "Japanese Journal of Applied Physics", 24, 6, 1985, pL. 387-390. In the following discussion, each of the angles such as the critical angle and glancing angle is measured as the angle the incident or emerging X-ray forms with the surface of the sample.
FIG. 20 shows how X-rays leaving the surface of a material to enter vacuum are refracted. As shown, the intensity of such X-rays decreases sharply if they are emitted at smaller angles than a certain critical value .theta..sub.c. The value of .theta..sub.c coincides with the critical angle for total reflection that occurs when X-rays of the same energy are launched from vacuum into the material of interest and it is expressed by: ##EQU1## where Z, A and .rho. are the atomic number, mass number and density, respectively, of the material of interest, and .lambda. is the wavelength of the incoming and outgoing X-ray.
Take, for example, the case where an electron beam is incident on the surface of the material. The characteristic X-rays are refracted as shown in FIG. 21 and those which are picked up at angles close to the critical angle .theta..sub.c contain information from the surface layer whereas those picked up at angles greater than .theta..sub.c contain information from deeper areas. As eq. (1) shows, the critical angle .theta..sub.c varies with the wavelength of incoming and outgoing X-ray and the shorter its wavelength (hence, the higher its energy), the smaller the critical angle is. In addition, the characteristic X-rays have energy values peculiar to the associated elements. For example, the intensity of characteristic X-rays emitted from the sample 21 having approximately one atomic layer of silver (Ag) evaporated on the Si surface has a pickup angle dependency as shown in FIG. 22. The characteristic X-rays emitted by Ag can be detected with a very high sensitivity by picking them up at the angle of total reflection of Ag characteristic X-rays with respect to Si or at nearby angles. In addition, the characteristic X-rays emitted from the neighborhood of the Si surface can be selectively detected by picking up the characteristic X-rays emitted by Si at the angle of total reflection of Si characteristic X-rays with respect to Si or nearby angles.
To take another example, the intensity of characteristic X-rays emitted from the sample 21 having calcium (Ca), iron (Fe) or copper (Cu) deposited as a trace impurity on the Si surface has a pickup angle dependency as shown in FIG. 23. Obviously, the angle of total reflection of emitted characteristic X-ray with respect to Si varies with the species of element that emits the X-ray. If the sample 21 is made of zinc sulfide (ZnS), the intensity of characteristic X-rays emitted from the sample has a pickup angle dependency as shown in FIG. 24. Obviously, the angle of total reflection of the Zn characteristic X-ray with respect to ZnS differs from that of the S characteristic X-ray with respect to ZnS and at the angle of total reflection of Zn, the S characteristic X-ray has a very small intensity whereas at the angle of total reflection of S, the Zn characteristic X-ray contains more of the information from a deep area of the sample.
A problem with the prior art elemental analyzer constructed in the manner described above, is that if more than one element is present in the neighborhood of the surface of the sample 21, the different values of energy possessed by X-rays characteristic of the individual elements provide different angles of total reflection for the sample 21. As a result, if the characteristic X-ray emitted by a certain element is detected at an angle in the neighborhood of the angle of total reflection for that element, the intensity of the characteristic X-ray of another element will decrease or more of the information from a deeper area will be detected to reduce the sensitivity of measurement for the surface of the sample (i.e., more of the unwanted information will be contained in information about the surface of the sample 21).
The prior art thin-film forming apparatus capable of compositional measurement of samples, which has the construction shown in FIG. 17, has had the following disadvantages:
(1) Conventionally, the X-ray transmissive window 7 is formed of a beryllium (Be) foil which, at a sufficient thickness to keep an ordinary degree of vacuum, will absorb soft X-rays having an energy of no more than 1 keV and this has made it difficult to measure the characteristic X-rays of materials such as oxides and carbides that contain light elements;
(2) Even if the X-ray transmissive window 7 is formed of an organic material that transmits soft X-rays (e.g. Parylene), the heat of radiation that is generated from the substrate and other sources during thin film formation can potentially soften or melt the X-ray transmissive window;
(3) Even if the apparatus is so modified as to enable the measurement of light elements, the efficiency of generation of characteristic X-rays decreases drastically with the decreasing atomic number of light elements such as oxygen (O), nitrogen (N) and carbon (C). With compounds containing such light elements, the yield of characteristic X-rays will vary considerably with the species of element even if the compositional ratio of any two elements is 1:1. Accordingly, in performing pulse measurements on the characteristic X-rays entering the detector 2, the counting rate of pulses is limited by the higher of the two yields of characteristic X-rays and the counting rate for the measurement of characteristic X-rays of the lower of the two yields becomes very low. Under the circumstances, the time of pulse measurement must be prolonged in order to achieve a higher precision of measurement. Since it takes a long time to accomplish the measurement of materials containing light elements, it is difficult to catch up with the high speed of thin film formation, so has it been to consecutively measure the composition of the sample 21 in the process of thin film formation if it contains light elements.